Don't bail out of loading when one library couldn't be found.

Also includes some cleanup to the error dialog (removing
sunken border, fixing OK button, etc.)

Fixes: lp:1749572
* https://bugs.launchpad.net/kicad/+bug/1749572

common/html_messagebox.cpp

@@ -35,7 +35,7 @@ HTML_MESSAGE_BOX::HTML_MESSAGE_BOX( wxWindow* parent, const wxString& aTitle) :
     ListClear();
 
     // Gives a default logical size (the actual size depends on the display definition)
-    SetSizeInDU( 240, 150 );
+    SetSizeInDU( 320, 120 );
 
     Center();
 }